What makes one marriage last and another fall apart in a few years?

According to a study of couples married more than 20 years, the answer, not too surprisingly, is commitment. What's more surprising, however, is that it is not so much the partners' commitment to each other that counts, as their shared commitment to the institution of marriage.

Dr. David Fenell, a marriage and family therapist at the University of Colorado at Colorado Springs, presented his findings last month at the annual meeting of the American Association for Marriage and Family Therapy in Chicago. Fenell surveyed 150 couples married for an average of 29 years -- to the same spouse.

He found that "a commitment to the institution of marriage," topped the list of essential characteristics that enabled a marriage to survive for decades. The couples said they felt that marriage itself was "an important ideal and concept."

First, Fenell and his colleagues developed a list of 70 characteristics that long-married couples said were crucial to making a marriage work. In addition, the couples identified critical issues that had tested their marriages -- and been overcome -- in order for their relationships to survive.

Fenell and his colleagues then took the characteristics and asked the 150 long-married couples to rank them in order of importance. Out of the list of 70 characteristics, six emerged in a distinct cluster as the most highly ranked by both husbands and wives.

After "commitment to marriage as an institution" came "commitment to the marriage partner," Fenell said, which included these qualities: loyalty and respect for the spouse, considering your husband or wife as your best friend, being able to forget and be forgiven when differences arise and the desire to please and support your partner.

The other most commonly reported characteristics of a strong marriage were (in descending order):

Strong moral values.

Desire to have children and to rear them well.

Maintaining a good sexual relationship (including sexual fidelity) with your spouse.

Spiritual commitment -- be it faith in God or commitment to some other religious idea that is bigger than the relationship itself.

Asked to list the critical issues that had been the most important to overcome for their marriage to survive, the couples most often ranked establishing and maintaining a mutually satisfactory sexual relationship.

Other key areas of dispute that had to be overcome included:

Disciplining and managing children. "Having different parenting styles can be a serious problem for a couple," Fenell says.

"When the kids are older, it may create a situation for the couple to look at themselves and say, 'I wonder what I did that may have caused this problem or that problem in my children,' " Fenell says. "The relationship is strained. It's a time when the couple can either pull apart or be supportive of each other."

The inability to recognize and deal with a spouse's thoughts and feelings. "It's a bit of a stereotype," Fenell said, "but men typically don't do as well with women's feelings as women would like."

Having unrealistic expectations of a spouse.

Taking the spouse for granted.

Having financial disagreements.

Not sharing important decisions.

Not sharing enough common interests or activities.

Loss of trust during some period in the relationship.

"If we can prepare couples to deal with these kinds of problems in marriage," Fenell said, "the big problem of long-term relationships lasting is really diminished."